Various optical detectors such as infrared radiation detectors are available in today's electronics industry. Many techniques for converting infrared radiation to visible images are also known. One such example of an infrared imager available in today's art includes a deflectable microelectromechanical (MEM) cantilever device formed of a bi-material on a semiconductor substrate. The bi-material portion of the micro-cantilever device is formed of two different materials sharing a contiguous surface, and having mismatched thermal coefficients of expansion (TCE). Examples of such bi-material MEM micro-cantilever devices and methods for forming the same, are as disclosed in U.S. Pat. No. 5,844,238 issued to Sauer et al. and U.S. Pat. Nos. 6,140,646 and 6,420,706, issued to Lurie et al.
The bi-material MEM micro-cantilever devices presently available in the art, bend, or deflect, when infrared radiation is absorbed upon an absorber element of the micro-cantilever heating the bi-material section of the micro-cantilever, thereby urging one of the bi-materials to expand at a greater rate than the other bi-material, thereby causing the micro-cantilever to deflect, or bend. The terms bend and deflect may be used interchangeably hereinafter.
When infrared radiation is incident upon such a micro-cantilever being used as an optical detector, it is desired to produce a visible image having an intensity which varies directly with the intensity of the incident infrared radiation. As a micro-cantilever device bends in response to such incident infrared radiation, it approaches a physical limitation to its degree of bending. For example, if a micro-cantilever device is fabricated so as to bend downward in response to incident infrared radiation, the physical limitation is reached when the micro-cantilever touches the substrate over which it is formed. For a micro-cantilever device chosen to bend upward in response to incident infrared radiation, this, too, will reach a physical limitation point past which it can no longer bend. As such, when this point of the physical limitation of bending is approached, the micro-cantilever device is more resistant to bending and therefore, less responsive to additional infrared radiation. An increased amount of incident infrared radiation will not cause the same extent of bending as when the micro-cantilever is in the rest position. While a significantly higher dose of infrared radiation may force the micro-cantilever to bend slightly more towards its physical limitation, the degree of this bending will not be proportional, so the device will not be linear in this region. Thus, the linear range of the device is limited.
Moreover, after the physical limitation point is reached, additional incident infrared radiation will simply not cause any further bending. This limits the dynamic range of the device. Since the intensity of an optical image ultimately produced from such a device, is based on the degree of bending, it can be seen that such a device having a poor dynamic range and limited linearity, produces an image having the same shortcomings.
Various methods for sensing the degree of bending are available in the art. Examples of such methods include optically measuring the distance between the micro-cantilever and the substrate, and electrically measuring the capacitance of a capacitor which includes an electrode formed in the substrate and another electrode formed in the micro-cantilever above the substrate. Various methods for producing a visible image having an intensity based upon the extent of bending are also known.